Alt-BEAM Archive

Message #09697



To: beam@sgiblab.sgi.com
From: cactus@dynamite.com.au
Date: Wed, 2 Feb 100 06:07:05 GMT
Subject: [alt-beam] Re: SIM D1 notes and questions


Thanks, Wilf.

That explains a lot and really increases my understanding of what's going on
with the circuit. I've been keeping an eye on your SIM D1 posts since you put
up the first schematic but I think I must have missed a couple. I hadn't
realised that it's really designed for low voltages.

I still have a couple of questions though. The chip I'm using is a 74HC14 so
with the Schmitt trigger wouldn't the threshold be 1/3 Vdd? Is oscillation such
a problem with Schmitt triggers?

I guess with a 74HC04 I could get the circuit to trigger before it gets so
dark? that might be preferable.

My storage cap array was made of two 1/3F gold caps in series placed in
parallel with a cheap 10,000uF cap (just because I started with the 10,000uF
and didn't bother taking it off when I decided to increase the capacitance).
Thinking that the cheap cap might be leaking and causing the problem, I took it
out of the circuit, which did seem to make some difference, though it wasn't a
complete solution.

I measured the current coming out of the 74HC14 and it only seemed to be about
10 uF - not in the mF range at all. However, even at this rate, I
calculate/estimate (a little of each) that two hours of dim light should be
enough to make a nasty dent in the stored charge. A long twilight such as we
have at this time of year will do it. Having the lights on in the house for a
few hours after sunset will also do it. That ties in well with empirical
observations of the circuit.

I could get around the problem by increasing the capacitance, but I don't want
the thing to make beep beep noises all night long - five minutes will be quite
long enough, thanks.

Other possible strategies I'm thinking of are:

1. Try a 4000 series chip instead. This isn't really an option since my only
local supplier (DSE) doesn't seem to carry a 4000 series inverter (or maybe I
just don't know what to look for which is just as likely - most of what I know
about electronics I've learned in the last two months). Besides, with only
0.15F storage capacitance, I'm not sure the reduced current leakage would
really help.

2. Try to reduce Vdd relative to the voltage on the cap, perhaps by using two
resistors as a voltage divider. The extra current leakage through the
resistors might eat up any gains made. Maybe a diode or three would do it. As
with the above option the reduced current leakage may not prevent the voltage
loss given the timeframe concerned.

3. Use the remaining 5 inverters (currently just sitting there with their
inputs tied to ground) to make a charge pump that will drive the chirper
circuit from a low voltage on the cap.

Option 3 sounds the best to me. However, I have to confess that looking at a
few of your charge pump circuits, I have no idea how they work and no idea how
to design an appropriate one.

>I did publish a slightly more complex SIMD1 design which
>uses positive feedback to help "push" the input voltage through the
>threshold region and clamp the solar cell output until the cap is almost
>fully discharged.

I'm not sure if I caught that post. If I did, I'm not sure I'd recognise it.
I'll have a look through the archives. Any hints on what to look for?

Thanks again, Wilf. It amazes me that you not only produce so many cool
circuits but also find time to provide "user support".

Simon.



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